1. Field of the Invention
The present invention relates to a system and method for controlling coating width in an electrode plate manufacturing process in which a paste containing an active material is applied to a core substrate to form a stripe-shaped coating is layer.
2. Description of the Related Art
In recent years, batteries have been increasingly used as a power source of various devices. Accordingly, there has been a strong demand for an increase in the capacity and output power of batteries and for a reduction in the thickness and weight thereof. Lithium ion rechargeable batteries, lithium polymer rechargeable batteries, nickel metal hydride batteries, and the like have been used as batteries capable of satisfying the demand. These batteries are being improved to further increase the capacity and output power and also to reduce the cost and stabilize the life.
As an electrode plate for such batteries, there is known an electrode plate constituted by forming a coating layer containing an active material on one or both sides of a metal core substrate serving as a collector. The coating layer is formed by coating the surface of the core substrate with a paste containing the active material. In order to reduce manufacturing cost in a method for manufacturing such an electrode plate, various methods have been known (see, for example, Japanese Patent Laid-Open Publication Nos. 2001-327906, 2005-183181, 2005-216722, and 2005-216723). Specifically, while a strip-like core substrate is fed in its longitudinal direction, a plurality of stripes of a coating layer are formed so as to be separated in the width direction is of the core substrate with an uncoated portion serving as a lead portion formed between the stripes. After the coating layer is dried and compressed, the core substrate is subjected to cutting, whereby an electrode plate having a desired size is manufactured with high productivity.
As a method for forming stripes of a coating layer on the surface of a core substrate, there is known, for example, a method described as a conventional example in Japanese Patent Laid-Open Publication No. 2001-327906. In this method, as shown in FIG. 6A, a masking tape 32 is applied to a plurality of uncoated portion formation portions on a strip-like core substrate 31. While the core substrate 31 is fed in its longitudinal direction, a paste 34 is ejected from a slit nozzle of a die 33 having a width approximately the same as that of the core substrate 31 to coat almost the entire surface of the core substrate 31 with the paste 34 as shown in FIG. 6B. After the coating layer is dried, the masking tapes 32 are peeled from the core substrate 31. In this manner, as shown in FIG. 6C, a plurality of stripes of a coating layer 35 is formed on the surface of the core substrate 31, and an uncoated portion 36 is formed between the stripes and in the side end portions of the core substrate 31.
According to this method for forming a coating layer, the coating layer 35 can be formed with high width accuracy by using the masking tape 32. However, the masking tape 32 must is be used, and steps of applying and peeling the masking tape 32 must be provided, thereby increasing cost. Furthermore, since the masking tape 32 is exposed to high temperature in a drying furnace, the following problems arise. Wrinkles are formed in the core substrate 31 due to thermal shrinkage of the masking tape 32. The dimensional accuracy of the coating layer 35 is adversely affected. The adhesion of the masking tape 32 is increased, and thus cracks and breakage are likely to occur in the core substrate 31 when the masking tape 32 is peeled.
Similarly, there are also known the following methods which employ a masking tape. In a method described in Japanese Patent Laid-Open Publication No. 2005-183181, in order to prevent the occurrence of wrinkles and distortions in a core substrate due to roll pressing (compression) of a coating layer, the coating layer is formed such that the entire coating surface is flat after drying. Specifically, in order to make the coating thickness of the coating layer uniform over the entire width including both side edge portions, coating is performed such that the coating thickness on masking tapes is different from that on coated portions. In a method described in Japanese Patent Laid-open Publication No. 2005-216722, a paste is applied in stripes so as not to be applied to portions where a masking tape is placed, and the masking tape is peeled after roll pressing. In a method described in Japanese Patent Laid-Open Publication No. 2005-216723, a multilayered masking tape is employed in which each layer can be peeled layer by layer. An upper layer of the masking tape is peeled such that the entire coated surface after coating is flat, and the masking tape is peeled after roll pressing. However, a masking tape must be used, and steps of applying and peeling the masking tape must be provided. Therefore, the problem of cost increase cannot be solved.
In order to solve the problem, a method for manufacturing an electrode plate is disclosed in Japanese Patent Laid-open Publication No. 2001-327906. In this method, as shown in FIG. 7A, a core substrate 41 is fed with the rear surface thereof supported by a supporting member 44. The core substrate 41 and an application apparatus 42 having a plurality of slit nozzles 43 are arranged such that a gap is provided between the surface of the core substrate 41 and the slit nozzles 43. In this state, a paste 45 is ejected from each of the slit nozzles 43 toward the surface of the core substrate 41, whereby an electrode plate 47 having a plurality of stripes of a coating layer 46 applied to the core substrate 41 is manufactured, as shown in FIG. 7B.
Moreover, there is known an apparatus for manufacturing an electrode plate (see Japanese Patent Laid-Open Publication No. 2002-25541). In this apparatus, in order to prevent unevenness in the weight per unit area of a coating layer formed by coating a core substrate with a paste, the weight of the coating layer is measured after drying, and the amount of the paste to be supplied to a coating apparatus is controlled based on the measurement results.
This apparatus for manufacturing an electrode plate is described with reference to FIG. 8. A strip-like core substrate 51 is fed from an unwinder 52 to a coating unit 53, and the core substrate 51 travels while being wound on a support roller 54 of the coating unit 53. During this state, a paste 56 is ejected from a coating apparatus 55 toward the surface of the core substrate 51 to form a coating layer. Subsequently, the core substrate 51 is fed to a drying furnace 57 to dry the coating layer and then passes through a weight measuring apparatus 58. At this time, a plurality of radiations, such as beta rays and X-rays, having different penetration depths are applied, and the weight per unit area of the coating layer is measured separately from the weight of the core substrate based on the amount of the transmitted rays. The core substrate is then wound by means of a winding apparatus 59. A pump 61 for supplying the paste to the coating apparatus 55 is controlled by a controlling unit 60 based on the results obtained by comparing a predetermined standard weight with the weight measured by a weight measuring apparatus 58, whereby the unevenness of the weight of the coating layer is suppressed within a predetermined range,
Meanwhile, as shown in FIGS. 7A and 7B, in the method in which the paste 45 is ejected from the coating apparatus 42 having the plurality of slit nozzles 43 to form the stripe-shaped coating layer 46 on the surface of the core substrate 41, the width of the coating layer 46 is basically controlled by the size of the slit nozzles 43. However, the width of the stripe-shaped coating layer 46 can change due to various causes of fluctuations, causing a problem that an electrode plate 47 having a coating layer 46 formed with high width accuracy is difficult to manufacture. For example, since the properties of the paste 45 are relatively easily changed, the weight per unit area of the coating layer is often changed even when the amount supplied from a supply pump is controlled to a constant value. Meanwhile, when the amount of the paste 56 supplied from the pump 61 is changed in order to make the weight per unit area of the coating layer uniform as shown in FIG. 8, the width of the coating layer is changed. Therefore, a problem arises in that it is difficult to accurately adjust the width of the coating layer to a constant value.